Mass air flow sensors have been utilized for many years with electronic fuel injection engines. For an internal combustion engine to run, it needs air and fuel mixed together ideally in a proper ratio. In electronic fuel injected engines, the engine control module (ECM) receives input related to how much air is used by an engine so that the correct amount of fuel can be injected.
Mass air flow meters (MAF) provides an output which is the input into the ECM relative to the amount of air directed into the engine. Mass air flow sensors normally convert the amount of air drawn into the engine into a voltage or frequency signal provided to the ECM. The mass air flow (MAF) sensor is usually located in the intake air stream between the air cleaner and the throttle body.
The basic operating principle of a MAF is as follows. A Wheatstone bridge is provided with the MAF such that an adjustable resistor, normally a thermistor, is utilized to sense temperature of incoming air. A second resistor is a sensor, such as a platinum hot wire, is normally heated to a constant temperature in relation to the thermistor. Increased air flow will cause the hot wire to lose heat faster. The MAF will compensate by sending more current through the wire. The MAF simultaneously measures the current flow and outputs a voltage or frequency signal in proportion to the current flow through the hot wire. The more air flow that flows over the sensor, the more voltage or higher frequency is typically sent.
As air temperature rises, the hot wire temperature will also need to rise. This adjustment is known as temperature compensation. For example, if the platinum hot wire is heated to 225° Celsius and the air temperature is at 25° Celsius, the difference is 200° Celsius. However, if the air temperature rises to 45° Celsius, the hot wire temperature also needs to rise to maintain the 200° difference so it would move the hot wire temperature to 245° Celsius through the temperature compensation process. A detailed description of temperature compensation is taught in U.S. Pat. No. 4,807,151 which is incorporated herein by reference which discusses temperature compensation.
A number of the manufacturers are providing mass air flow meters to market. The constraints of these prior art system prevent the MAF from changing temperature compensation, (it is fixed) under changing conditions. U.S. Pat. No. 4,807,151 provides for an ability to change gain to at least a portion of the output in an analog manner, but no ability is provided to affect the output either before generating the output or afterwards based on changing conditions. Furthermore, the thermistor in addition to the platinum hot wire is required by the circuit. Finally, laser trimming such as that taught in U.S. Pat. No. 5,493,892 (herein incorporated by reference) discusses the laser calibration of the two resistors which are not thermistors in the bridge is typically required. The laser trimming process is time intensive and the thermistor is relatively expensive, thereby increasing the cost of providing the mass air flow sensor.
Decreasing the cost, eliminating the laser trimming step, and/or other affects would be desirable for many applications. Accordingly, there exists a need for an improved mass air flow sensor for these and other reasons.